1. Field of the Invention
The present invention relates to an optical semiconductor device module and bidirectional module, used for a semiconductor laser, a semiconductor photodiode, or a semiconductor optical modulator.
2. Background Art
A conventional optical semiconductor device module is configured such that: a ball lens is held by a cap having a light transmissive hole; and the light emitted from the semiconductor device mounted on a stem is converged by the ball lens and then coupled to the optical fiber.
For example, when the ball lens is formed of BK-7 (borosilicate glass) and has a refractive index of 1.5 and a diameter of 1.5 mm, if the distance from the end face of the semiconductor device to (the center of) the light entrance surface of the ball lens is 0.83 mm, the distance from the stem to the focal point of the exiting light is 6.5 mm. In this case, the optical coupling efficiency to single-mode fiber having a mode diameter of 10 μm is approximately 16%.
Further, when the ball lens is formed of TaF3 (dense lanthanum flint glass) and has a refractive index of 1.78 and a diameter of 2.0 mm, if the distance from the end face of the semiconductor device to (the center of) the light entrance surface of the ball lens is 0.53 mm, the distance from the stem to the focal point of the exiting light is 6.6 mm. In this case, the optical coupling efficiency to single-mode fiber having a mode diameter of 10 μm is approximately 20%.
Further, Japanese Laid-Open Patent Publication No. 8-130320 (1996) discloses an optical system which achieves a coupling efficiency of 30% or more by using a ball lens 0.6 mm-1.4 mm in diameter having a refractive index of 1.9.
With these conventional optical semiconductor device modules, however, if the assembling accuracy of the semiconductor device or the ball lens is degraded to approximately ±50 μm or worse, the optical coupling efficiency to the optical fiber significantly drops. To prevent this from happening, it is necessary to adjust the installation position of the optical fiber according to the angle of the light emitted from the semiconductor device or to employ an aspherical lens or a plurality of lenses, which has been an obstacle to cost reduction.